1. Field of the Invention
The invention relates to generally spherical, conductive, inherently tacky, elastomeric, solvent-dispersible, solvent-insoluble polymeric microparticles which may be used in applications requiring electrical conductivity or elimination of electrostatic charge. The ionically conductive polymeric microparticles may be included in adhesive formulations which provide adhesive tapes with a remarkable capacity for preventing static charge build-up.
2. Description of the Art
Antistatic adhesive compositions are useful for attaching surface-mount components at points on printed circuit boards where they are to be conductively attached e.g., by soldering. Antistatic adhesives, when coated on selected substrates and suitably converted, provide antistatic, pressure-sensitive adhesive tapes. The tapes are useful for masking printed circuit boards prior to wave soldering and the like.
A number of methods are known for preparing antistatic adhesive compositions. One common method is the addition of conductive moieties to conventional adhesive formulations. Antistatic species may be introduced as conductive materials such as electrically conductive metal or carbon particles. Compositions of this type are disclosed in prior art references including EP 0276691A, EP 0518722A, U.S. Pat. No. 4,606,962, EP 0422919A, U.S. Pat. Nos. 3,104,985, 4,749,612 and 4,548,862.
The addition of ionic materials to reduce generation of static charge is also known. Suitable materials of this type include ion conducting species such as those disclosed in Japanese patents JP 61,272,279 and JP 63,012,681.
U.S. Pat. No. 4,098,945 discloses a conductive composition which comprises a polymeric binder system, a plurality of insoluble spherical domains dispersed in the system, and at least one electrically conductive filler dispersed in the binder which provides conductive pathways through the composition. The spherical domains are preferably adhesive microspheres, the use of which lessens the amount of conductive filler used.
Yet another type of antistatic tape material is provided using a metal foil tape backing. One example of this, disclosed in U.S. Pat. No. 3,497,383, provides embossed foil tapes where contact points of metal project from the surface of the adhesive.
Several problems are encountered with known antistatic adhesive compositions. Compositions which rely on electrically conductive metallic particles require the particles to be in contact with each other and with the surfaces to be protected from static charge. Since the particles are surrounded by insulating adhesive binder, this may be difficult to achieve. In materials exhibiting antistatic behavior due to ionic conductivity, effective elimination of static charge may be severely affected by low humidity environments. For example, the biomedical electrodes of publication WO 92/10553 and antistatic coatings of European Application EP 399441A are not useful unless sufficient moisture is available.
Surprisingly, the present invention does not use conductive particles derived from metals or carbon nor does it require high humidity levels to exert its capacity for suppressing the build-up of static charges.
Instead, it is believed that the unique properties of materials of the present invention are provided by the use of polymeric microparticles having complexes of polymer electrolytes on the surface of each microparticle.
Complexes of polyethylene oxide (PEO) and lithium salts have been shown to be promising materials as solid state polymer electrolytes. The use of these materials the development of high energy lithium batteries is considered by Gilmour et al in Proc. Electrochemical Society, 89-94, (1989). Lithium salts, like those disclosed in WO 8,303,322, U.S. Pat. No. 4,471,037 and FR 2,568,574, are most commonly used with PEO in polymer electrolytes. Other metal salts such as alkaline earth salts may also enhance electrolytic properties as described in U.S. Pat. No. 5,162,174. Applications for polymer electrolytes have expanded from a focus on energy storage batteries to their use in other areas such as electrochromic displays and addition to molding resins in the production of conductive molded articles.
Disclosures of polymer electrolytes have dealt with their use as films or as additives to other resin matrices which benefit from the ability to conduct electric current. In no case has there been reference to the preparation of microparticles containing ionically conducting species as part of the microparticles. Further, a problem with polymer electrolytes in film form is their poor performance at room temperature.
It is possible to overcome such problems associated with existing antistatic pressure-sensitive adhesive tapes and polymer electrolytes by using compositions of the invention. Polymeric microparticles having polymer electrolytes on the surface of each polymer particle provide antistatic particles which may be incorporated into adhesive compositions.
When coated on suitable substrates and converted into tape format, these adhesives provide antistatic tapes which are extremely effective in dissipating electrostatic charge.
Particulate adhesives are also known in the art, and have been coated on a variety of substrates and used primarily in applications requiring a low level of adhesion, e.g., repositionability. Such spheres and their use in aerosol adhesive systems having repositionable properties are disclosed in U.S. Pat. No. 3,691,140 (Silver). These microparticles are prepared by aqueous suspension polymerization of alkyl acrylate monomers and ionic comonomer, e.g., sodium methacrylate, in the presence of an emulsifier. The use of a water-soluble, substantially oil-insoluble ionic comonomer is critical to preventing coagulation or agglomeration of the microparticles. However, particulate adhesives disclosed in the prior art have all been useful as repositionable adhesives for such applications as Post-It.TM. brand notes, and other removable items. Pressure-sensitive tapes made with this type of adhesive are likely to be considered unsuitable for use as antistatic tapes due to their ease of removal.
However, it has now been discovered that it is possible to develop a balance of properties whereby surface conductive polymer particles may be formulated into adhesives which possess adhesion which is sufficient for permanent attachment, and low tribocharging characteristics.